If the stench from your cat's litter box is a bit too much for you to take, consider rigging together an exhaust system using a 120mm computer case fan, a 12vdc power supply, 4" flexible dryer duct, a few steel tomato cans, and other bits found in most households. This fan is designed to operate by switch, but you can also add an Arduino relay switch to turn it on and off by motion.

The version we're covering today by Instructables user astilly has the advantages of lower cost, easier to find parts, and the ability to run the system as always on or add Arduino and extra components to turn the fan off when the cats are using the litter box.

Full instructions and step-by-step photos can be found at the source link below.